vary header can't be overwritten even defined explicitly in middleware

[yanv1991]

Link to the code that reproduces this issue or a replay of the bug

https://github.com/yanv1991/cache-control/

To Reproduce

• start application in prod mode.
• open browser devtools in the root page
• go to network tab and look for the html document at the very top level, click on it
• look for response headers (vary)

Current vs. Expected behavior

current result:

• vary is not being set(overwritten), even when we set this in middleware file using dynamic rendering

expected result

• vary should be defined in response header according to the value set in middleware file

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
      Platform: darwin
      Arch: x64
      Version: Darwin Kernel Version 20.6.0: Thu Jul  6 22:12:47 PDT 2023; root:xnu-7195.141.49.702.12~1/RELEASE_X86_64
    Binaries:
      Node: 16.14.2
      npm: 8.5.0
      Yarn: 1.22.19
      pnpm: N/A
    Relevant Packages:
      next: 13.4.20-canary.28
      eslint-config-next: 13.4.19
      react: 18.2.0
      react-dom: 18.2.0
      typescript: 5.2.2
    Next.js Config:
      output: N/A

Which area(s) are affected? (Select all that apply)

App Router

Additional context

we need to define this vary to be passed to our cdn in order to cache effectively

[simonri]

I think this is intentional. As a workaround you can create a patch using patch-package.

Edit the following files and add your values to RSC_VARY_HEADER.

/node_modules/next/dist/client/components/app-router-headers.js
/node_modules/next/dist/client/components/app-router-headers.d.ts
/node_modules/next/dist/esm/client/components/app-router-headers.js

Link to the file

[yanv1991]

Thanks @simonri, would be nice to see this restriction in the docs, or at least mentioning the reason of doing that

[github-actions]

This closed issue has been automatically locked because it had no new activity for 2 weeks. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.